1. The Field of the Invention
The present invention relates to techniques for the disposal of coal fines and/or coarse coal refuse from conventional coal preparation plants. More particularly, the present invention relates to an improved method and system for the disposal of waste coal products in the form of low cost, low density coal water slurry by co-firing the slurry in cyclone-fired boilers in such a manner as to effect a reduction in nitrogen oxides emissions, thereby minimizing landfill requirements and improving the fuel recovery efficiency of coal preparation plants.
2. The Relevant Technology
The use of coal water slurry as a fuel is well known, but historically in the United States, with minor exceptions, the use of coal water slurry has been confined primarily to demonstration projects. The disadvantages of using coal water slurry in the past have been related to the energy required to evaporate the water in the slurry, and the relatively high price of preparing and transporting a high coal density (65% plus coal) coal water slurry fuel. However, with new coal recovery techniques, waste coal from coal preparation plants that has been impounded in ponds or in coarse refuse piles may now be recovered economically and prepared as coal water slurry fuels. The recovered coal water slurry fuel in most cases will be less expensive than the processed coal sold from these preparation plants.
It is estimated that two to four billion tons of coal fines are impounded in ponds throughout the United States, with coarse refuse doubling the total coal portion ending up as waste products from these coal preparation plants. These waste materials are an environmental liability for the coal preparation plant operators and are a lost fuel source.
Various techniques have been developed previously to utilize coal/water mixtures in combustion systems. For example, in U.S. Pat. No. 3,589,314 to Tratz et al., a method and device for pressure spraying and burning a 60/40 weight ratio coal dust/water mixture is described. Complete combustion of the coal dust/water mixture in a furnace is accomplished by heating the mixture under pressure prior to spraying the mixture through a nozzle with radial or axial bores into a combustion chamber. Additionally, U.S. Pat. No. 4,444,126 to Forster describes an apparatus for combustion of a suspension of coal particles in water, in which combustion air preheated to 550.degree. C. (1022.degree. F.) is forced into a coal water slurry preheated to 100.degree. C. (212.degree. F.) in a portion of the burner upstream of the burner flame. In U.S. Pat. No. 4,465,495 to Scheffee, a process is disclosed for making a high heating value coal water slurry fuel that can be injected directly into a furnace as a combustible fuel for the express purpose of replacing fuel oil in oil-fired boilers.
Furthermore, U.S. Pat. No. 5,380,342 to Leonard et al. describes a process of co-firing coal water slurry specifically in a gas fired boiler through wall-fired gas burners. The furnace is preheated using natural gas and is then fired with approximately 60% of the heat input supplied by pulverized coal and 40% of the heat input supplied by natural gas. The gas is gradually shut off completely while coal water slurry is added such that the heat input ratio of pulverized coal to coal water slurry is 80/20. The process of this patent is specifically directed to a modified gas fired boiler wherein the burners are designed to fire three fuels: natural gas, coal water slurry, and pulverized coal. Further, when co-firing in wall fired units, most of the ash produced from coal combustion (some 80%) is carried out of the boiler with the flue gas. This means that the ash content of the slurry must be kept at a minimal level, since gas fired units are not equipped to handle fly ash.
In U.S. Pat. No. 5,513,583 to Battista, a coal water slurry burner assembly is disclosed in which a metal tube for conveying a suspension of fine-grained coal in a liquid or aqueous slurry is inserted through a conventional air burner for combustion of pulverized coal. The metal tube is attached to a nozzle oriented and adapted to conically spray the aqueous slurry into the pulverized coal fueled flame of the burner.
The methods disclosed in the above patents accomplish their intended purpose to facilitate the combustion of coal water slurry in fossil fuel-fired boilers, but are not methods specifically designed to eliminate waste coal products from coal preparation plants or to minimize nitrogen oxides emissions. An improved process is needed that will allow for the beneficial use of all preparation plant waste coal by-products as fuels, eliminating the environmental impact of landfilling, and also providing a technique to effect up to 100% fuel recovery efficiency for coal preparation plants.